Catalyst degradation diagnosis device and diagnosis method for internal combustion engine

ABSTRACT

The oxygen storage capacity preceding a previous oxygen storage capacity and the previous oxygen storage capacity of each of an upstream catalyst and a downstream catalyst are measured, and a present oxygen storage capacity of the downstream catalyst is measured. Then, sulfur poisoning of the upstream catalyst and the downstream catalyst is detected on the basis of an oxygen storage capacity change amount of each of the upstream catalyst and the downstream catalyst from the oxygen storage capacity preceding the previous one to the previous oxygen storage capacity, and the oxygen storage capacity change amount of the downstream catalyst from the previous oxygen storage capacity to the present oxygen storage capacity. The presence/absence of the sulfur poisoning can be accurately detected by utilizing the difference between the manners of sulfur poisoning of the two catalysts.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2007-314921 filed on Dec. 5, 2007 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device that diagnoses the presence/absence of degradation of a catalyst disposed in an exhaust passageway of an internal combustion engine, and to a method for such diagnosis.

2. Description of the Related Art

For example, in internal combustion engines for vehicles, the exhaust system is provided with a catalyst for purifying exhaust gas. Among such catalysts, there is a catalyst that has an oxygen storage capability (O₂ storage capability). This type of catalyst adsorbs and retains excess oxygen present in exhaust gas when the air-fuel ratio of exhaust gas flowing into the catalyst becomes larger than a stoichiometric air-fuel ratio (stoichiometric value), that is, becomes lean in fuel, and then releases the adsorbed and retained oxygen when the air-fuel ratio of the exhaust gas flowing into the catalyst becomes smaller than the stoichiometric value, that is, becomes rich in fuel. For example, in a gasoline engine, the air-fuel ratio control is performed so that the exhaust gas flowing into the catalyst has an air-fuel ratio in the vicinity of the stoichiometric value. If a three-way catalyst having an oxygen storage capability is used, deviations of the actual air-fuel ratio from the stoichiometric value, which can occur depending on the operation condition, can be absorbed to some extent due to the oxygen storage/release action of the three-way catalyst.

If the catalyst degrades, the exhaust gas purification efficiency of the catalyst declines. The degree of degradation of the catalyst and the degree of decline of the oxygen storage capability of the catalyst have a correlation since both the degradation of the catalyst and the decline of the oxygen storage capability of the catalyst are results of reactions via noble metals. Therefore, degradation of the catalyst can be detected by detecting a decline of the oxygen storage capability. In a method generally employed to this end, an active air-fuel ratio control of forcing a switch between a rich state and a lean state of the air-fuel ratio of exhaust gas flowing into the catalyst is performed. In conjunction with the execution of the active air-fuel ratio control, the oxygen storage capacity of the catalyst is measured in order to diagnose the presence/absence of degradation of the catalyst.

Depending on the area of use of the vehicle, the fuel can contain a relative high concentration of sulfur (S). In the case where such a fuel is fed into the vehicle, there occurs a poisoning (sulfur poisoning) in which the performance of the catalyst declines due to the effect of sulfur components in exhaust gas. If the sulfur poisoning occurs, the oxygen storage/release action of the catalyst is impeded, so that the oxygen storage capacity of the catalyst declines. However, if a fuel low in the sulfur concentration is fed into the vehicle again, the poisoned state will soon be dissolved. The performance decline of the catalyst due to the sulfur poisoning is temporary and recoverable. In the diagnosis regarding degradation of the catalyst, it is necessary to avoid falsely diagnosing a temporary degradation caused by the sulfur poisoning as an unrecoverable permanent degradation (thermal degradation), which the diagnosis is intended to find. In particular, it is necessary to avoid false diagnosis as degradation with regard to the catalyst that is still normal while being in the proximity of the border (criteria) between normality and degradation.

As a countermeasure against this, it is conceivable to perform a process in which when the catalyst is sulfur-poisoned, the air-fuel ratio is transitionally corrected so as to desorb sulfur, and after that, the diagnosis regarding degradation is performed. This forced desorption of sulfur in this manner has a problem of further degrading the catalyst.

According to vigorous studies of the present inventors, it has been found that in the case where an upstream catalyst and a downstream catalyst are disposed in the exhaust passageway of the internal combustion engine, the manner of the sulfur poisoning is different between the upstream catalyst and the downstream catalyst. Therefore, if the sulfur poisoning of at least one of the upstream and downstream catalysts can be detected by utilizing the foregoing, finding, the misdiagnosis as mentioned above can be prevented, and advantage can be achieved in the improvement of the diagnosis accuracy and reliability.

SUMMARY OF THE INVENTION

In view of the foregoing circumstances, the invention has been accomplished, and provides a catalyst degradation diagnosis device and a catalyst degradation diagnosis method for an internal combustion engine equipped with an upstream catalyst and a downs catalyst disposed in an exhaust passageway of the internal combustion engine which can detect the sulfur poisoning of at least one of the two catalysts and improve the diagnosis accuracy and reliability.

According to one aspect of the invention, there is provided a catalyst degradation diagnosis device for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst, the device being characterized by including: a measurement device that measures an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and a sulfur poisoning detection device that detects sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from an oxygen storage capacity of the upstream catalyst preceding a previous oxygen storage capacity of the upstream catalyst to the previous oxygen storage capacity, an oxygen storage capacity change amount of the downstream catalyst from an oxygen storage capacity of the downstream catalyst preceding a previous oxygen storage capacity of the downstream catalyst to the previous oxygen storage capacity, and an oxygen storage capacity change amount of the downstream catalyst from the previous oxygen storage capacity to a present oxygen storage capacity of the downstream catalyst, when the oxygen storage capacity of each of the upstream catalyst and the downstream catalyst preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured by the measurement device, and the present oxygen storage capacity of the downstream catalyst has been measured by the measurement device.

Besides, according to another aspect of the invention, there is provided a catalyst degradation diagnosis method for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst. This diagnosis method includes the steps of: measuring an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and detecting sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from an oxygen storage capacity of the upstream catalyst preceding a previous oxygen storage capacity of the upstream catalyst to the previous oxygen storage capacity, an oxygen storage capacity change amount of the downstream catalyst from an oxygen storage capacity of the downstream catalyst preceding a previous oxygen storage capacity of the downstream catalyst to the previous oxygen storage capacity, and an oxygen storage capacity change amount of the downstream catalyst from the previous oxygen storage capacity to a present oxygen storage capacity of the downstream catalyst, when the oxygen storage capacity of each of the upstream catalyst and the downstream catalyst preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured, and the present oxygen storage capacity of the downstream catalyst has been measured.

As a result of vigorous study by the present inventors, it turned out that the decline of the oxygen storage capacity of a catalyst caused by the sulfur component in fuel is attributed to the following two phenomena. In a first phenomenon, SOx in exhaust gas reacts with the catalyst component supported in a catalyst, that is, a noble metal, and therefore lowers the reaction rate of the noble metal, whereby the oxygen storage capacity of the catalyst is lowered. In a second phenomenon, SOx in exhaust gas adsorbs to a storage component of the catalyst, whereby the oxygen storage capacity of the catalyst is lowered. In the case of the upstream catalyst, the catalyst temperature thereof is higher than that of the downstream catalyst Therefore, the second phenomenon, that is, the sulfur adsorption onto the storage component, is difficult to occur, and the decline of the oxygen storage capacity of the upstream catalyst is predominantly caused by the first phenomenon, that is, the decline of the reaction rate of the noble metal. The decline of the oxygen storage capacity occurs immediately after the fuel is changed from a low-sulfur fuel to a high-sulfur fuel (i.e., immediately after the refueling with a high-sulfur fuel). On the other hand, in the case of the downstream catalyst, the first phenomenon and the second phenomenon occur so that the oxygen storage capacity declines, and the decline of the oxygen storage capacity gradually progresses after the change of the fuel. Thus, the manners of sulfur poisoning of the two catalysts are different from each other.

Considering the difference, the invention can suitably detect the sulfur poisoning of the upstream catalyst and the downstream catalyst. For example, the oxygen storage capacity change amount of the upstream catalyst from the measurement of oxygen storage capacity preceding the previous one to the previous measurement, and the oxygen storage capacity change amount of the downstream catalyst from the measurement preceding the previous one to the previous measurement are larger than predetermined values, it is tentatively estimated that there is a possibility of sulfur poisoning. Then, if the oxygen storage capacity change amount of the downstream catalyst from the previous measurement to the present measurement is larger than a predetermined value, it is finally detected that the two catalyst have sulfur poisoning. As for the upstream catalyst, the second phenomenon is difficult to occur, and therefore, if the oxygen storage capacity declined in the period during the measurement preceding the previous one to the previous measurement, the oxygen storage capacity does not exhibit a large change after that, that is, in the present measurement. On the other hand, as for the downstream catalyst, the second phenomenon occurs as well as the first phenomenon, and therefore, the oxygen storage capacity, after declining, does not stop declining but further declines. Utilizing this difference, the sulfur poisoning of a catalyst can be accurately detected, and the diagnosis accuracy and reliability can be improved.

Besides, it is also preferable to determine the presence/absence of degradation of the upstream catalyst and the downstream catalyst based on the oxygen storage capacity measured, and prohibit determination regarding the degradation when the su poisoning of the upstream catalyst and the downstream catalyst is detected. This makes it possible to prevent false determination of the presence of degradation of a catalyst based on a measured oxygen storage capacity value that has declined due to sulfur poisoning.

Besides, it is also preferable that the oxygen storage capacity preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst be a value measured during a trip preceding a previous trip and a value measured during the previous trip, respectively, which are before and after refueling. Since the decline of the oxygen storage capacity due to sulfur poisoning occurs immediately after the fuel is changed, that is, after refueling, it is preferable to check a change in the oxygen storage capacity between before and after refueling.

Besides, according to still another aspect of the invention, there is provided a catalyst degradation diagnosis device for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst, the device being characterized by including: a measurement device that measures an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and a sulfur poisoning detection device that detects sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity of the upstream catalyst to a present oxygen storage capacity of the upstream catalyst, and an oxygen storage capacity change amount of the downstream catalyst from a previous oxygen storage capacity of the downstream catalyst to a present oxygen storage capacity of the downstream catalyst, when the previous oxygen storage capacity and the present oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured by the measurement device.

Besides, according to yet still another aspect of the invention, there is provided a catalyst degradation diagnosis method for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst. This diagnosis method includes the steps of: measuring an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and detecting sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity of the upstream catalyst to a present oxygen storage capacity of the upstream catalyst, and an oxygen storage capacity change amount of the downstream catalyst from a previous oxygen storage capacity of the downstream catalyst to a present oxygen storage capacity of the downstream catalyst, when the previous oxygen storage capacity and the present oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured.

If sulfur poisoning occurs on the upstream catalyst and the downstream catalyst, the oxygen storage capacities of the two catalysts measured first following change of the fuel decline greatly. Hence, the sulfur poisoning of the upstream catalyst and the downstream catalyst can also be detected on the basis of the oxygen storage capacity change amount of the upstream catalyst from the previous measurement to the present measurement and the oxygen storage capacity change amount of the downstream catalyst from the previous measurement to the present measurement.

According to a further aspect of the invention, there is provided a catalyst degradation diagnosis device for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst, the device being characterized by including: a measurement device that measures an oxygen storage capacity of the upstream catalyst; and a sulfur poisoning detection device that detects sulfur poisoning of the upstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity to a present oxygen storage capacity, when the previous oxygen storage capacity and the present oxygen storage capacity have been measured by the measurement device.

Besides, according to a still further aspect of the invention, there is provided a catalyst degradation diagnosis method for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst. This diagnosis method includes the steps of measuring an oxygen storage capacity of the upstream catalyst; and detecting sulfur poisoning of the upstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity to a present oxygen storage capacity, when the previous oxygen storage capacity and the present oxygen storage capacity have been measured.

In the case where an upstream catalyst and a downstream catalyst are disposed in the exhaust passageway of an internal combustion engine, the refueling with a high-sulfur fuel lowers the oxygen storage capacity, particularly, of the upstream catalyst, immediately after the change of the fuel. Hence, utilizing this, the sulfur poisoning of the upstream catalyst can be detected on the basis of the oxygen storage capacity change amount of the upstream catalyst from the previous measurement to the present measurement.

Thus, the invention achieves an excellent advantage as follows. That is, in the case where the exhaust passageway of an internal combustion engine is equipped with an upstream catalyst and a downstream catalyst, the sulfur poisoning of at least one of the catalysts can be detected, and therefore the diagnosis accuracy and reliability can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic diagram showing a construction of an embodiment of the invention;

FIG. 2 is a schematic sectional view showing a construction of a catalyst shown in FIG. 1;

FIG. 3 is a time chart regarding a first mode of a catalyst degradation diagnosis;

FIG. 4 is a time chart similar to FIG. 3 for describing a measurement method for an oxygen storage capacity;

FIG. 5 is a time chart regarding a second mode of We catalyst degradation diagnosis;

FIG. 6 is a graph showing a change in the oxygen storage capacity of an upstream catalyst between before and after the change from a fuel to another in accordance with a first embodiment;

FIG. 7 a flowchart showing a catalyst degradation diagnosis process in accordance with the first embodiment;

FIG. 8 is a flowchart showing another catalyst degradation diagnosis process in accordance with the first embodiment;

FIG. 9 is a graph showing a change in the oxygen storage capacity of the upstream catalyst between before and after the change of a fuel to another in accordance with a second embodiment;

FIG. 10 is a graph showing changes in the oxygen storage capacity of a downstream catalyst in accordance with the second embodiment;

FIG. 11 is a flowchart showing a catalyst degradation diagnosis process in accordance with the second embodiment;

FIG. 12 is a flowchart showing a second catalyst degradation diagnosis process in accordance with the second embodiment; and

FIG. 13 is a flowchart continuing from FIG. 12, showing the second catalyst degradation diagnosis process in accordance with the second embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Best modes for carrying out the invention will be described hereinafter with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing a construction of an embodiment of the invention. As shown in FIG. 1, an internal combustion engine 1 generates power by burning a mixture of fuel and air within combustion chambers 3 formed in a cylinder block 2 and thereby reciprocating pistons 4 within the combustion chambers 3. The internal combustion engine 1 is a multi-cylinder engine (only one cylinder is shown) for a vehicle, and is a spark ignition type internal combustion engine and, more specifically, a gasoline engine.

A cylinder head of the internal combustion engine 1 is provided with intake valves Vi that open and close intake ports, and exhaust valves Ve that open and close exhaust ports. The intake valves Vi and the exhaust valves Ve are opened and closed by camshafts (not shown). Besides, in a top portion of the cylinder head, ignition plugs 7 for igniting mixture within combustion chambers 3 are attached separately for each cylinder.

The intake ports of the individual cylinders are connected to a surge tank 8 that is an intake assembly chamber, via branch pipes that correspond to the individual cylinders. An intake pipe 13 that forms an intake assembly passageway is connected to an upstream side of the surge tank 8. An air cleaner 9 is provided on an upstream end of the intake pipe 13. An air flow meter 5 for detecting the amount of intake air, and an electronically controlled throttle valve 10 are incorporated in the intake pipe 13 in that order from the upstream side. Incidentally, the intake ports, the surge tank 8 and the intake pipe 13 form an intake passageway.

Injectors (fuel injection valves) 12 that inject fuel into the intake passage, particularly, into the intake ports, are provided separately for each cylinder. The fuel injected from each injector 12 is mixed with intake air to form a mixture, which is in turn taken into a corresponding one of the combustion chambers 3 when the corresponding intake valve Vi opens, and is then compressed by the piston 4, and is ignited to burn by the ignition plug 7.

On the other hand, the exhaust ports of the individual cylinders are connected to an exhaust pipe 6 that forms an exhaust assembly passageway via branch pipes of the cylinders. An upstream catalyst 11 and a downstream catalyst 19 each of which is formed by a three-way catalyst that has an oxygen storage capability are attached in series to the exhaust pipe 6. Incidentally, the exhaust ports, the branch pipes and the exhaust pipe 6 form an exhaust passageway. The upstream catalyst 11 is attached at a position that is relatively near to the combustion chambers 3, so as to be activated in an early period by utilizing exhaust heat. On the other hand, the downstream catalyst 19 is attached at a position relatively far from the combustion chambers 3, for example, under a floor of the vehicle, or the like.

A pre-catalyst sensor 17 is disposed at an upstream side of the upstream catalyst 11, and an inter-catalyst sensor 18 is disposed between the upstream catalyst 11 and the downstream catalyst 19, and a post-catalyst sensor 21 is disposed at a downstream side of the downstream catalyst 19. Each of the pre-catalyst sensor 17, the inter-catalyst sensor 18 and the post-catalyst sensor 21 is an air-fuel ratio sensor for detecting the air-fuel ratio of exhaust gas. In particular, the pre-catalyst sensor 17 is made up of a so-called wide-range air-fuel ratio sensor, and is therefore able to continuously detect the air-fuel ratio over a relatively wide range, and outputs a signal having a value proportional to the detected air-fuel ratio. On other hands, the inter-catalyst sensor 18 and the post-catalyst sensor 21 are each made up of a so-called O₂ sensor, and has a characteristic in which the output value sharply changes at the stoichiometric air-fuel ratio.

The ignition plugs 7, the throttle valve 10, the injectors 12, etc. are electrically connected to an electronic control unit (hereinafter, abbreviated as “ECU”) 20 as a control device. The ECU 20 includes a CPU, a ROM, a RAM, input/output ports, memory devices, etc. (none of which is shown). The ECU 20 is also electrically connected to the air flow meter 5, the pre-catalyst sensor 17, the inter-catalyst sensor 18, and the post-catalyst sensor 21, and furthermore to a crank angle sensor 14 that detects the crank angle of the internal combustion engine 1, an accelerator operation amount sensor 15 that detects the accelerator operation amount, and other various sensors, via A/D converters or the like (not shown). The ECU 20 controls the ignition plugs 7, the throttle valve 10, the injectors 12, etc., and therefore controls the ignition timing, the amount of fuel injection, the fuel injection timing, the throttle opening degree, etc. so that a desired output of the engine is obtained, on the basis of the detection values from various sensors, and the like.

Each of the upstream catalyst 11 and the downstream catalyst 19 removes NOx, HCs and CO simultaneously when the air-fuel ratio A/F of the exhaust gas that flows into the catalyst is equal to the stoichiometric air-fuel ratio (stoichiometric value, for example, A/Fs=14.6). In response to this, the ECU 20 controls the air-fuel ratio of mixture so that during usual operation of the internal combustion engine, the air-fuel ratio of the mixture discharged from the combustion chambers 3 which flows into the upstream catalyst 11, that is, the pre-catalyst air-fuel ratio A/Ffr, becomes equal to the stoichiometric air-fuel ratio. Concretely, the ECU 20 sets a target air-fuel ratio A/Ft equal to the stoichiometric air-fuel ratio, and feedback-controls the amount of fuel injection injected from the injectors 12 so that the pre-catalyst air-fuel ratio A/Ffr detected by -the pre-catalyst sensor 17 becomes equal to the target air-fuel ratio A/Ft. Therefore, the air-fuel ratio of the exhaust gas that flows into the catalyst 11 is kept in the vicinity of the stoichiometric air-fuel ratio, so that the catalyst 11 delivers its maximum purification performance. In this air-fuel ratio feedback control, a main feedback control of making the pre-catalyst air-fuel ratio A/Ffr equal to the stoichiometric air-fuel ratio, and a subsidiary feedback control of making the inter-catalyst air-fuel ratio A/Fmd detected by the inter-catalyst sensor 18 equal to the stoichiometric air-fuel ratio are performed. The subsidiary feedback control is performed for the purpose of eliminating the deviation of the center air-fuel ratio caused by degradation of the pre-catalyst sensor 17 or the like.

Herein, the upstream catalyst 11 and the downstream catalyst 19, which are the objects of the degradation diagnosis, will be described in more detail. Since the upstream catalyst 11 and the downstream catalyst 19 have the same construction, the upstream catalyst 11 will be taken as an example in the description below. As shown in FIG. 2, in the catalyst 11, a surface of a support base (not shown) is coated with a coating material 31, and a catalyst component 32 in a fine powder state is retained in a large quantity on the coating material 31 in a dispersed arrangement, and is exposed within the catalyst 11. The catalyst component 32 is mainly made up of a noble metal, such as Pt, Pd, etc., and serves as active sites for reactions of exhaust gas components, such as NOx, HCs and CO. On the other hand, the coating material 31 contains an oxygen storage component that plays a role of a promoter that accelerates the reactions on the interface between exhaust gas and the catalyst component 32, and that is capable of absorbing and releasing oxygen according to the air-fuel ratio of the atmosphere gas. The oxygen storage component is made up of, for example, cerium dioxide CeO₂, or zirconia. For example, if the atmosphere gas around the catalyst component 32 and the coating material 31 is richer than the stoichiometric air-fuel ratio, oxygen stored in the oxygen storage component present around the catalyst component 32 is released therefrom. As a result, the released oxygen oxidizes unburnt components, such as HCs and CO, thus removing the components. Conversely, if the atmosphere gas around the catalyst component 32 and the coating material 31 is leaner than the stoichiometric air-fuel ratio, the oxygen storage component present around the catalyst component 32 absorbs oxygen from the atmosphere gas, resulting in the reductive removal of NOx.

Due to this oxygen storage/release action, the three exhaust gas components, that is, NOx, HCs and CO, can be simultaneously removed although the exhaust air-fuel ratio may fluctuate to some extent about the stoichiometric air-fuel ratio during an ordinary air-fuel ratio control. Therefore, it is also possible to perform exhaust gas purification by intentionally oscillating the pre-catalyst air-fuel ratio A/Ffr to small extents about the stoichiometric air-fuel ratio and therefore repeatedly performing the storage and the release of oxygen. It is to be understood that “storage” used herein means retention of a substance (solid, liquid, gas, molecules) in the form of at least one of adsorption, adhesion, absorption, trapping, occlusion, and others.

By the way, in the catalyst 11 in a brand-new state, the catalyst component 32 in a fine particle state as described above is uniformly arranged in a large quantity in a dispersed fashion, so that a state of high probability of contact between exhaust gas and the catalyst component 32 is maintained. However, as the catalyst 11 degrades over time due to thermal stress, disappearance of a portion of the catalyst component 32 is observed, and some particles of the catalyst component 32 are thermally solidified by exhaust heat, thus entering a sintered state (see broken lines in FIG. 2). This brings about a decline in the probability of contact between exhaust gas and the catalyst component 32, and becomes a cause of lowering the purification rate. Besides this, the amount of the coating material 31 present around the catalyst component 32, that is, the amount of the oxygen storage component, decreases, and the oxygen storage capability declines.

The degree of degradation of the catalyst 11 and the degree of decline of the oxygen storage capability of the catalyst 11 are in a correlation. Therefore, in this embodiment, the degree of degradation of the catalyst 11 is detected by detecting the oxygen storage capability of the catalyst 11. Incidentally, the oxygen storage capability of the catalyst 11 is represented by the magnitude of the oxygen storage capacity (OSC, that is, the O₂ storage capacity in the unit of g) that is a maximum amount of oxygen that can be stored by the catalyst 11 as it is at the present time.

Hereinafter, the catalyst degradation diagnosis in this embodiment will be described. Incidentally, for the sake of convenience, a mode of the degradation diagnosis (first mode) in which the diagnosis is limited to the upstream catalyst 11, which has a great influence on the emission quality, will firstly be described, and a mode of the degradation diagnosis (second mode) in which the downstream catalyst 19 is also included as an object of the diagnosis will be described later.

The catalyst degradation diagnosis of this embodiment is basically a diagnosis based on the foregoing Cmax method. At the time of the degradation diagnosis of the catalyst 11, the ECU 20 executes an active air-fuel ratio control In the active air-fuel ratio control, the air-fuel ratio of mixture and, therefore, the pre-catalyst air-fuel ratio A/Ffr is alternately switched between the rich side and the lean side of a predetermined center air-fuel ratio A/Fc in a forced fashion (actively). Incidentally, the air-fuel ratio given by a change to the fuel-rich side will be termed the rich air-fuel ratio A/Fr, and the air-fuel ratio given by a change to the fuel-lean side will be termed the lean air-fuel ratio A/Fl. When the pre-catalyst air-fuel ratio A/Ffr is changed to the rich side or the lean side by the active air-fuel ratio control, the oxygen storage capacity OSC of the catalyst 11 is measured.

The degradation diagnosis of the catalyst 11 is executed when the internal combustion engine 1 is in a steady operation and the catalyst 11 is within an activation temperature range. As for the measurement of the temperature of the catalyst 11 (catalyst bed temperature), a temperature sensor may be used for direct detection In this embodiment, however, the temperature of the catalyst 11 is estimated from the state of operation of the internal combustion engine. For example, the ECU 20 estimates the temperature Tc of the catalyst 11, utilizing a pre-set map, on the basis of the intake air amount Ga detected by the air flow meter 5. Incidentally, the parameters used to estimate the catalyst temperature may also include parameters other than the intake air amount Ga, for example, the engine rotation speed Ne (rpm), etc.

FIGS. 3A and 3B show the output from the pre-catalyst sensor 17 and the output of the inter-catalyst sensor. 18 at the time of execution of the active air-fuel ratio control, respectively, by solid lines. Besides, in FIG. 3A, the target air-fuel ratio A/Ft generated within the ECU 20 is shown by a broken line. The output values of the pre-catalyst sensor 17 and the inter-catalyst sensor 18 are correspond to values of the pre-catalyst air-fuel ratio A/Ffr and the inter-catalyst air-fuel ratio A/Fmd, respectively.

As shown in FIG. 3A, the target air-fuel ratio A/Ft is forced to alternately switch between an air-fuel ratio (rich air-fuel ratio A/Fr) that is apart from the stoichiometric air-fuel ratio (stoichiometric value) A/Fs serving as a center air-file ratio to the rich side by a predetermined amplitude (rich-side amplitude Ar, where Ar>0) and an air-fuel ratio (lean air-fuel ratio A/Fl) that is apart from the stoichiometric air-fuel ratio to the lean side by a predetermined amplitude (lean-side amplitude Al, where Al>0). Then, following the switching of the target air-fuel ratio A/Ft, the pre-catalyst air-fuel ratio A/Ffr as the actual value switches with a small time delay relative to the target air-fuel ratio A/Ft. From this, it can be understood that the target air-fuel ratio A/Ft and the pre-catalyst air-fuel ratio A/Ffr are equivalent to each other, except that there is a time delay therebetween.

In the example shown in FIGS. 3A and 3B, the rich-side amplitude Ar and the lean-side amplitude Al are substantially equal. For example, the stoichiometric air-fuel ratio A/Fs=14.6, the rich air-fuel ratio A/Fr=14.1, the lean air-fuel ratio A/Fl=15.1, and the rich-side amplitude Ar=the lean-side amplitude Al=0.5. Compared with the case of usual air-fuel ratio control, the amplitude of the air-fuel ratio in the case of the active air-fuel ratio control is large, that is, the values of the lean-side amplitude Al in the control are large.

Incidentally, the timing at which the target air-fuel ratio A/Ft is switched is a timing at which the output of the inter-catalyst sensor 18 switched from the rich side to the lean side or from the lean side to the rich side. As shown in the diagrams, the output voltage of the inter-catalyst sensor 18 sharply changes at the stoichiometric air-fuel ratio A/Fs. That is, when the inter-catalyst air-fuel ratio A/Fmd is smaller than the stoichiometric air-fuel ratio A/Fs, that is, on the rich side thereof, the output voltage of the inter-catalyst sensor 18 becomes equal to or greater than a rich-side criterion value VR. When the inter-catalyst air-fuel ratio A/Fmd is larger than the stoichiometric air-fuel ratio A/Fs, that is, on the lean side thereof, the output voltage of the inter-catalyst sensor 18 becomes less than or equal to a lean-side criterion value VL. Herein, VR>VL, for example, VR=0.59 (V), and VL=0.21 (V).

As shown in FIGS. 3A and 3B, when the output voltage of the inter-catalyst sensor 18 changes from a value on the rich side to the lean side to become equal to the lean-side criterion value VL (time t1), the target air-fuel ratio A/Ft is switched from the lean air-fuel ratio A/Fl to the rich air-fuel ratio A/Fr. After that, when the output voltage of the inter-catalyst sensor 18 changes from a value on the lean side to the rich side to become equal to the rich-side criterion value VR (time t2), the target air-fuel ratio A/Ft is switched from the rich air-fuel ratio A/Fr to the lean air-fuel ratio A/Fl.

While the active air-fuel ratio control of performing the changing of the air-fuel ratio as described above is executed, the oxygen storage capacity OSC of the catalyst 11 is measured in the following manner to determine whether or not the catalyst 11 has degraded.

Referring to FIGS. 3A and 3B, before time t1, the target air-fuel ratio A/Ft is set at the lean air-fuel ratio A/Fl, and a fuel-lean gas flows into the catalyst 11. At this time, the catalyst 11 continues absorbing oxygen. However, when oxygen is absorbed to the fill capacity, no more oxygen can be absorbed, so that the lean gas passes through the catalyst 11, and flows out at the downstream side of the catalyst 11. If this happens, the inter-catalyst air-fuel ratio A/Fmd changes to the lean side. At the time point (t1) at which the output voltage of the inter-catalyst sensor 18 reaches the lean-side criterion value VL, the target air-fuel ratio A/Ft is switched to the rich air-fuel ratio A/Fr, or is inverted. In this manner, the target air-fuel ratio A/Ft is inverted, with the output of the inter-catalyst sensor 18 serving as a trigger.

Then, a rich gas flows into the catalyst 11. At this time, the catalyst 11 continues releasing the oxygen that has been stored. Therefore, exhaust gas of substantially the stoichiometric air-fuel ratio A/Fs flows out at the downstream side of the catalyst 11, so that the inter-catalyst air-fuel ratio A/Fmd does not become fuel-rich and therefore the output of the inter-catalyst sensor 18 does not become inverted. As oxygen continues to be released from the catalyst 11, the oxygen stored in the catalyst 11 is entirely released. From that time point on, no more oxygen can be released, and therefore fuel-rich gas passes through the catalyst 11, and flows out at the downstream side of the catalyst 11. Hence, the inter-catalyst air-fuel ratio A/Fmd changes to the rich side. At the time point (t2) at which the output voltage of the inter-catalyst sensor 18 reaches the rich-side criterion value VR, the target air-fuel ratio A/Ft is switched to the lean air-fuel ratio A/Fl.

The greater the oxygen storage capacity OSC, the longer the time during which oxygen can continue to be absorbed or released. That is, in the case where the catalyst has degraded, the inversion cycle of the target air-fuel ratio A/Ft (e.g., the time from t1 to t2) becomes longer. As the degradation of the catalyst advances, the inversion cycle of the target air-fuel ratio A/Ft becomes shorter.

Therefore, utilizing this, the oxygen storage capacity OSC is measured as follows. As shown in FIGS. 4A and 4B, immediately after the target air-fuel ratio A/Ft is switched to the rich air-fuel ratio A/Fr as the time t1, the pre-catalyst air-fuel ratio A/Ffr as the actual value switches to the rich air-fuel ratio A/Fr with a slight delay. From the time point t11 at which the pre-catalyst air-fuel ratio A/Ffr reaches the stoichiometric air-fuel ratio A/Fs till the time point t2 at which the target air-fuel ratio A/Ft becomes inverted, the oxygen storage capacity dOSC (the instantaneous value of the oxygen storage capacity) at every predetermined small amount of time is calculated, and the oxygen storage capacity dOSC at every predetermined small amount of time is integrated from the time t11 to the time t2. In this manner, the oxygen storage capacity, that is, the released amount of oxygen (OSC(1) in FIG; 4A), in the present oxygen release cycle is measured.

dOSC=ΔA/F×Q×K=|A/Ffr−A/Fs|×Q×K   (1)

In the equation, Q is the amount of fuel injection. The short fall or excess of air with respect to the stoichiometric value can be calculated by multiplying the air-fuel ratio difference ΔA/F by the fuel injection amount Q. In addition, K is a constant that represents the proportion of the oxygen (about 0.23) contained in air.

Basically, the oxygen storage capacity OSC obtained by one measurement process is used, that is, compared with a predetermined degradation criterion value OSCs in order to determine whether or not the catalyst has degraded. That is, if the measured oxygen storage capacity OSC is greater than the degradation criterion value OSCs, it is determined that the catalyst is normal. If oxygen storage capacity OSC is less than or equal to the degradation criterion value OSCs, it is determined that the catalyst has degraded. However, in order to improve accuracy in this embodiment, the oxygen storage capacity (amount of oxygen stored in this case) is also measured in the oxygen storage cycle during which the target air-fuel ratio A/Ft is on the lean side, and an average value of these oxygen storage capacities is measured as one unit of oxygen storage capacity in accordance with one storage-release cycle. Furthermore, the storage-release cycle is repeated a plurality of times to obtain values of a plurality of units of oxygen storage capacity, and the average value of the obtained values is used as a final measured oxygen storage capacity value.

As for the measurement of the oxygen storage capacity (stored oxygen amount) in the oxygen storage cycle, after the target air-fuel ratio A/Ft is switched to the lean air-fuel ratio A/Fl at a time t2 as shown in FIG. 4, an oxygen storage capacity dOSC at every predetermined small amount of time is calculated using the foregoing equation (1), and the oxygen storage capacity dOSC at every predetermined small amount of time is integrated, during a period from a time point t21 at which the pre-catalyst air-fuel ratio A/Ffr reaches the stoichiometric air-fuel ratio A/Fs to a time point t3 at which the target air-fuel ratio A/Ft is next inverted to the rich side. In this manner, the oxygen storage capacity OSC in the oxygen storage cycle, that is, the stored oxygen amount (OSC(2) in FIG. 4), is measured. The oxygen storage capacity OSC1 measured during the previous cycle and the oxygen storage capacity OSC2 measured during the present cycle should become substantially equal values.

Next, using the measured value of the oxygen storage capacity, determination regarding the degradation of the catalyst is performed. That is, the measured value of the oxygen storage capacity OSC is compared with a predetermined degradation criterion value OSCs. If the value of the oxygen storage capacity OSC is greater than the degradation criterion value OSCs, it is determined that the catalyst is normal. If the value of oxygen storage capacity OSC is less than or equal to the degradation criterion value OSCs, it is determined that the catalyst has degraded. In addition, in the case where it is determined that the catalyst has degraded, it is preferable to activate a warning device, such as a check lamp, in order to inform a user of that fact What has been described above is a basic content of a mode (first mode) of the degradation diagnosis in the case where the determination is limited to the upstream catalyst 11.

Incidentally, the method of the first mode is also applicable singly to the downstream catalyst 19. In this case, the output of the pre-catalyst sensor 17 is substituted by the output of the inter-catalyst sensor 18, and the output of the inter-catalyst sensor 18 is substituted by the output of the post-catalyst sensor 21.

Next, a mode (second mode) of the degradation diagnosis that includes the upstream catalyst 11 and the downstream catalyst 19 will be described. In the second mode, the switch timing of the target air-fuel ratio A/Ft in the active air-fuel ratio control is made to coincide with the inversion timing of the post-catalyst sensor 21 on the downstream side of the downstream catalyst 19, and the storage or release of oxygen is performed simultaneously and serially with respect to the upstream catalyst 11 and the downstream catalyst 19, the oxygen storage capacity OSC of the upstream catalyst 11 and the oxygen storage capacity OSC of the downstream catalyst 19 are simultaneously and serially measured.

FIGS. 5A to 5E show how various values change during execution of the active air-fuel ratio control. FIG. 5A shows the target air-fuel ratio A/Ft, FIG. 5B shows the output of the inter-catalyst sensor 18, FIG. 5C shows the actual inter-catalyst air-fuel ratio A/Fmd, FIG. 5D shows the output of the post-catalyst sensor 21, and FIG. 5E shows the post-catalyst air-fuel ratio A/Frr.

In the example shown in FIGS. 5A to 5E, prior to a time T1, the target air-fuel ratio A/Ft (and the pre-catalyst air-fuel ratio A/Ffr)) is kept at a rich air-fuel ratio, and the oxygen stored in the upstream catalyst 11 is released to purity exhaust gas, so that an exhaust gas of a substantially stoichiometric air-fuel ratio flows out at the downstream end of the upstream catalyst 11. Therefore, during that time, the inter-catalyst air-fuel ratio A/Fmd is kept substantially at the stoichiometric air-fuel ratio. Then, after the oxygen stored in the upstream catalyst 11 is entirely released, a rich gas containing unburned components flows out at the downstream end of the upstream catalyst 11. Corresponding to this, the output of the inter-catalyst sensor 18 becomes inverted from the lean side to the rich side. In the example shown in FIGS. 5A to 5E, the output of the inter-catalyst sensor 18 reaches the rich-side criterion value VR at the time T1.

However, unlike the first mode, the target air-fuel ratio A/Ft is not switched at this time point. Therefore, after the time T1, an exhaust gas of a rich air-fuel ratio flows into the downstream catalyst 19. As such a rich gas flows in, the downstream catalyst 19 purifies the rich gas while releasing the oxygen stored therein. Thus, an exhaust gas of a substantially stoichiometric air-fuel ratio flows out at the downstream end of the downstream catalyst 19. Therefore, during that time, the post-catalyst air-fuel ratio A/Frr is substantially maintained at the stoichiometric air-fuel ratio.

In the course of time, when the oxygen stored in the downstream catalyst 19 is entirely released, rich gas flows out at the downstream end of the downstream catalyst 19. Corresponding to this, the output of the post-catalyst sensor 21 becomes inverted from the lean side to the rich side. In the example shown, the output of the post-catalyst sensor 21 reaches the rich-side criterion value VR at the time 12. At the time point at which the output of the post-catalyst sensor 21 becomes inverted to the rich side, it can be determined that both the upstream catalyst 11 and the downstream catalyst 19 have completely released the oxygen stored therein. Therefore, simultaneously, the target air-fuel ratio A/Ft is switched to a lean air-fuel ratio so that the pre-catalyst air-fuel ratio A/Ffr becomes inverted to the lean side. Thus, at the timing at which the output of the post-catalyst sensor 21 becomes inverted, the target air-fuel ratio A/Ft is switched.

From the time T2 on, exhaust gas of lean air-fuel ratio flows into the upstream catalyst 11, so that the upstream catalyst 11 purifies exhaust gas while storing excess oxygen in the exhaust gas. Therefore, after a slight delay time elapses from the time T2, the inter-catalyst air-fuel ratio A/Fmd changes to the vicinity of the stoichiometric air-fuel ratio. Then, after oxygen is stored in the upstream catalyst 11 to its full capacity, lean gas flows out at the downstream end of the upstream catalyst 11. Corresponding to this, the output of the inter-catalyst sensor 18 becomes inverted from the rich side to the lean side. In the example shown in FIGS. 5A to 5E, the output of the inter-catalyst sensor 18 reaches the lean-side criterion value VL at a time T3.

As described above, at this time point, the target air-fuel ratio A/Ft cannot be switched yet. Then, from the time T3 on, exhaust gas of lean air-fuel ratio flows into the downstream catalyst 19. As fuel-lean gas flows in, the downstream catalyst 19 purifies the lean gas while storing excess oxygen. Therefore, exhaust gas of a substantially stoichiometric air-fuel ratio flows out at the downstream end of the catalyst, and the post-catalyst air-fuel ratio A/Frr is kept substantially at the stoichiometric air-fuel ratio.

In the course of time, when oxygen is stored in the downstream catalyst 19 to its full capacity, lean gas flows out at the downstream end of the downstream catalyst 19. Corresponding to this, the output of the post-catalyst sensor 21 becomes inverted from the rich side to the lean side. In the example shown in FIGS. 5A to 5E, the output of the post-catalyst sensor 21 reaches the lean-side criterion value VL at a time T4. At the time point at which the output of the post-catalyst sensor 21 becomes inverted to the lean side, it can be determined that both the upstream catalyst 11 and the downstream catalyst 19 have stored oxygen to their fuel capacities. Therefore, simultaneously with this, the target air-fuel ratio A/Ft is switched to the rich air-fuel ratio so that the pre-catalyst air-fuel ratio A/Ffr becomes inverted to the rich side.

Then, likewise, a rich gas flows into the upstream catalyst 11, so that the upstream catalyst 11 released the oxygen stored. Then, after the upstream catalyst 11 has completely released oxygen, rich gas flows out at the downstream end of the upstream catalyst 11, so that the output of the inter-catalyst sensor 18 becomes inverted to the rich side (time T5). At this time point, the target air-fuel ratio A/Ft is not switched.

As a result, rich gas flows into the downstream catalyst 19, so that the downstream catalyst 19 releases oxygen. Then, after the downstream catalyst 19 has completely released oxygen, rich gas flows out at the downstream end of the downstream catalyst 19, so that the output of the post-catalyst sensor 21 becomes inverted to the rich side (time T6). Simultaneously with this inversion, the target air-fuel ratio A/Ft is switched to the lean side.

Along with the active air-fuel ratio control as described above, the oxygen storage capacities OSC1, OSC2 of the upstream catalyst 11 and the downstream catalyst 19 are measured on the basis of the foregoing equation (1), as in the first mode. That is, when oxygen is stored into the upstream catalyst 11 during the time T2 to T3, the oxygen storage capacity dOSC at every predetermined small amount of time is calculated using the equation (1), and the oxygen storage capacity dOSC at every predetermined small amount of time is integrated from the time point at which the output of the pre-catalyst sensor 17 changes to the lean side and reaches a value substantially equivalent to the stoichiometric air-fuel ratio until the time point T3 at which the output of the inter-catalyst sensor 18 becomes inverted to the lean side, as in the example shown in FIGS. 4A and 4B. At this time, the value of the air-fuel ratio difference ΔA/F used is a difference between the stoichiometric air-fuel ratio and the air-fuel ratio converted from the output of the pre-catalyst sensor 17.

Subsequently, when oxygen is stored into the downstream catalyst 19 during the time T3 to T4, the oxygen storage capacity dOSC at every predetermined small amount of time is calculated using the equation (1), and the oxygen storage capacity dOSC at every predetermined small amount of time is integrated from the time point T3 at which the output of the inter-catalyst sensor 18 becomes inverted to the lean side until the time point T4 at which the output of the post-catalyst sensor 21 becomes inverted to the lean side. At this time, since lean gas passes through the upstream catalyst 11 and flows into the downstream catalyst 19, the value of the air-fuel ratio difference ΔA/F used is a difference between the stoichiometric air-fuel ratio and the air-fuel ratio converted from the output of the pre-catalyst sensor 17.

After that, when the target air-fuel ratio A/Ft is switched to the rich air-fuel ratio, the oxygen storage capacity of the upstream catalyst 11 is measured during the time T4 to T5, and the oxygen storage capacity of the downstream catalyst 19 is measured during the time T5 to T6 by substantially the same method as described above.

The measurement data about the oxygen storage capacity measured as described above are stored in the ECU 20, and are subjected to the above-described averaging process to calculate final measured oxygen storage capacity values OSC1, OSC2 of the upstream catalyst 11 and the downstream catalyst 19. Then, these measured values OSC1, OSC2 are compared with degradation criterion values OSC1 s, OSC2 s, respectively, which are individually set beforehand. According to results of the magnitude comparison, the normality/degradation of the catalyst is determined separately for each of the upstream catalyst 11 and the downstream catalyst 19. Incidentally, in the case where at least one of the catalysts is determined as being degraded, it is preferable to activate a warning device, for example, a check lamp or the like, in order to inform a user of the thus determined fact.

Next, detection of the sulfur poisoning of the catalysts in the embodiment will be described. As described above, in the case where a high-sulfur concentration fuel is used, there is a problem of the sulfur poisoning of a catalyst. If the sulfur poisoning of a catalyst occurs, the oxygen storage capacity of the catalyst temporarily declines, leading to a misdiagnosis or the like. As a result of vigorous study by the present inventors, it turned out that the upstream catalyst 11 and the downstream catalyst 19 experience the sulfur poisoning in different manners.

That is, the following two phenomena of the oxygen storage capacity of a catalyst declining due to the sulfur component in fuel exist. In a first phenomenon, SOx in exhaust gas reacts with the catalyst component 32 supported in a catalyst, that is, a noble metal, and therefore lowers the reaction rate of the noble metal, whereby the oxygen storage capacity of the catalyst is lowered. The decline of the oxygen storage capacity occurs immediately after the fuel is changed from a low-sulfur fuel to a high-sulfur fuel (i.e., immediately after the refueling with a high-sulfur fuel). The amount of decline is dependent on the sulfur concentration of the fuel and the magnitude of the oxygen storage capacity that the catalyst has. In a second phenomenon, SOx in exhaust gas adsorbs to the storage component (coating material) 31 of the catalyst, whereby the oxygen storage capacity of the catalyst is lowered. The decline of the oxygen storage capacity in this phenomenon gradually progresses after the fuel is changed from a low-sulfur fuel to a high-sulfur fuel.

With attention focused on the upstream catalyst 11, high-temperature exhaust gas from the combustion chamber 3 initially and constantly flows into the upstream catalyst 11, and therefore the temperature of the catalyst is high. Therefore, the second phenomenon, that is, the sulfur adsorption onto the storage component 31, is difficult to occur, and the decline of the oxygen storage capacity of the upstream catalyst 11 is predominantly caused by the first phenomenon, that is, the decline of the reaction rate of the noble metal.

On the other hand, with regard to the downstream catalyst 19, since the catalyst temperature thereof is lower than that of the upstream catalyst 11, the second phenomenon occurs as well as the first phenomenon, lowering the oxygen storage capacity. That is, while the oxygen storage capacity of the upstream catalyst 11 sharply declines immediately following the change of fuel, and subsequently remains substantially unchanged, the oxygen storage capacity of the downstream catalyst 19 continues to gradually decline after sharply declining following the change of fuel.

Therefore, in this embodiment, in view of the difference in the manner of sulfur poisoning, the sulfur poisoning of each catalyst is detected, and the degradation determination regarding a catalyst is prohibited as follows.

Firstly, a first embodiment in which the object of the degradation diagnosis is limited to the upstream catalyst 11 will be described. FIG. 6 shows a change in the oxygen storage capacity OSC1 of the upstream catalyst 11 between before and after the fuel is changed to a high-sulfur fuel. In the description herein, the “present” is synonymous with the present time, and means or indicates a timing or a period that serves as a reference. The “previous” indicates a timing or a period that immediately precedes the “present” timing or period. The “preceding the previous” indicates a timing or a period that immediately precedes the “previous” timing or period. The “trip” is a period of time during which the engine (or the engine system) is on. In the example shown in FIG. 6, the fuel is changed during an engine-off period between the previous trip and the present trip. What is shown by a solid line in FIG. 6 is an actual change in the oxygen storage capacity.

As show in FIG. 6, during the previous trip, the oxygen storage capacity OSC1 n-1 (previous OSC) of the upstream catalyst 11 was measured, and during the present trip, the oxygen storage capacity OSC1 n (present OSC) of the upstream catalyst 11 is measured From the previous oxygen storage capacity OSC1 n-1, the present oxygen storage capacity OSC1 n immediately following the change of the fuel greatly changes, more concretely, greatly declines. Hence, if the amount of change or the amount of decline from the previous oxygen storage capacity OSC1 n-1 to the present oxygen storage capacity OSC1 n is calculated, the calculated value can be used as a basis to detect that a high-sulfur fuel has been fed into the fuel tank or the like and also detect that the upstream catalyst 11 has been poisoned with sulfur. The oxygen storage capacity change amount (or decline amount) ΔOSC1 is defined herein simply as a difference between the previous oxygen storage capacity OSC1 n-1 and the present oxygen storage capacity OSC1 n, that is, ΔOSC1=OSC1 n-1−OSC1 n. However, other defines are also possible. For example, oxygen storage capacity change amount ΔOSC1 may also be defined as ΔOSC1=(OSC1 n-1−OSC1 n)/OSC1 n-1.

A catalyst degradation diagnosis process according to the first embodiment will be described with reference to FIG. 7. This process is executed by the ECU 20.

Firstly in step S101, it is determined whether or not a precondition for starting the diagnosis has been satisfied. The precondition is satisfied, for example, if the engine is in a steady operation state and the catalyst 11 and the air-fuel ratio sensors 17, 18, 21 have reached their predetermined activation temperatures. The steady operation state of the engine may be considered to be present, for example, if the fluctuation widths of the intake air amount Ga and the engine rotation speed Ne are within their respective predetermined ranges. Incidentally, the precondition is not limited to the foregoing examples. If the precondition has not been satisfied, the process ends. On the other hand, if the precondition has been satisfied, the process proceeds to step S102.

In step S102, the oxygen storage capacity OSC1 of the upstream catalyst 11 is measured. Since the diagnosis object is only the upstream catalyst 11 herein, it is preferable to adopt an oxygen storage capacity measurement method as described above in conjunction with the first mode shown in FIGS. 3A, 3B, 4A and 4B. However, it is also permissible to adopt an oxygen storage capacity measurement method as described above in conjunction with the second mode shown in FIGS. 5A to 5E and use only the measured oxygen storage capacity values of the upstream catalyst 11.

Subsequently in step S103, it is determined whether or not at least one of misfire and air-fuel ratio abnormality has been detected, concretely, whether or not such a detection history exists. That is, this embodiment is provided with an abnormality detection device that detects at least one of misfire and air-fuel ratio abnormality. For example, when fluctuations of the engine rotation speed Ne are larger than a predetermined value, or when a greatly rich air-fuel ratio is detected by the pre-catalyst sensor 17, or when the hydrogen concentration in exhaust gas is higher than a predetermined value in the case where a hydrogen sensor is provided in the exhaust passageway upstream of the upstream catalyst, it can be determined that misfire has occurred. Besides, for example, when the correction value of the feedback air-fuel ratio control is maintained at a value that is such as to correct the air-fuel ratio to the rich side and that is a predetermined limit value, it can be determined that there has occurred an air-fuel ratio abnormality because of insufficient supply of the fuel caused by a failure in the fuel system.

In the case where a poor-quality fuel or an unsuitable kind of fuel (e.g., diesel) or the like is fed, there sometimes occurs abnormality combustion or misfire causing a melt loss of a catalyst. Therefore, in step S103, it is determined whether or not there has occurred a factor of catalyst melt loss. If an affirmative determination is made in step S103, a usual-degradation determination is performed in step S107. That is, the oxygen storage capacity OSC1 of the upstream catalyst 11 measured in step S102 is compared with the degradation criterion value OSC1 s. If OSC1>OSC1 s, it is determined that the upstream catalyst 11 is normal. If OSC1≦OSC1 s, it is determined that the upstream catalyst 11 has degraded. In the case of a melt loss of the catalyst, which is a permanent degradation that is unrecoverable, the degradation determination is performed as usual, and the degradation thereof, it so determined, is indicated to a user by turning on a check lamp so as to prompt the user to replace the catalyst.

On the other hand, if a negative determination is made in step S103, the process proceeds to step S104, in which the oxygen storage capacity change amount ΔOSC1 of the upstream catalyst 11 is calculated. That is, using the oxygen storage capacity OSC1 n measured in step S102 in the present process and the oxygen storage capacity OSC1 n-1 measured in step S102 in the previous process, the oxygen storage capacity change amount ΔOSC1 is calculated as in ΔOSC1=OSC1 n-1−OSC1 n.

Next, the oxygen storage capacity change amount ΔOSC1 is compared with a predetermined threshold value α (see FIG. 6). If ΔOSC1≦α, the process proceeds to step S107, in which the degradation determination is performed on the basis of the measured oxygen storage capacity value OSC1 of the upstream catalyst 11 measured in step S102, as usual.

On the other hand, when ΔOSC1>α, which means that the oxygen storage capacity OSC1 of the upstream catalyst 11 has greatly changed (declined), the process proceeds to step S106. In step S106, it is determined that the sulfur poisoning of the upstream catalyst 11 is present, and simultaneously the degradation determination based on the measured oxygen storage capacity value OSC1 of the upstream catalyst 11 is prohibited. Thus, the sulfur poisoning of the upstream catalyst 11 is suitably detected, and the degradation determination based on the measured oxygen storage capacity value OSC1 that has declined due to the sulfur poisoning is prohibited, whereby the misdiagnosis can be prevented and the diagnosis accuracy and reliability can be improved.

Next, another catalyst degradation diagnosis process according to the first embodiment of will be described with reference to FIG. 8. This process is executed by the ECU 20. The same steps as those in the process in FIG. 7 are merely represented by the reference numerals obtained by changing the third digit from the right in each step number from “1” to “2”, and detailed description of those steps are omitted. The following description is given mainly on the differences from the process shown in FIG. 7.

This another process is different from the foregoing process in that step S202A is added subsequently to step S202, and step S208 is added subsequently to step S206. After step S202, step S202A is executed. In step S202A, it is determined whether or not the measurement of the oxygen storage capacity in step S202 is the first measurement of the oxygen storage capacity performed after the fuel has been changed. Since the decline of the oxygen storage capacity caused by the sulfur poisoning of the upstream catalyst 11 suddenly occurs immediately after the change of the fuel, it is advantageous for the improvement of the diagnosis accuracy to add information regarding the presence/absence of change of the fuel. In this case, a detection device that detects the presence/absence of the execution of the change of the fuel is provided and, for example, the detection device has a remaining amount sensor for the fuel tank (fuel gage) or a sensor that detects the opening of the refill lid. In the case where it is detected that the remaining amount of fuel in the fuel tank has increased or that the refill lid has been opened, it is considered that the change of the fuel has been performed.

If a negative determination is made in step S202A, the process proceeds to step S207, in which the usual degradation determination is performed. On the other hand, if an affirmative determination is made in step S202A, the process of steps S203 to S208 is performed, in which the presence/absence of the feeding of a high-sulfur fuel is detected, and therefore the presence/absence of the sulfur poisoning of the upstream catalyst 11 is detected. That is, in this another process, the detection of the sulfur poisoning of the upstream catalyst 11 is executed only at the time of the first measurement of the oxygen storage capacity performed after the change of the fuel.

Besides, after in step S206, the sulfur poisoning of the upstream catalyst 11 is detected, and the degradation determination regarding the upstream catalyst 11 is prohibited, the process proceeds to step S203, in which the check lamp is turned on, and simultaneously a diagnosis code indicating the sulfur poisoning and the like is recorded in the ECU 20. This promotes the user to have the feeding of a low-sulfur fuel again. Alternatively, at a later stage in the maintenance of the vehicle, it can be made known to a service person that the upstream catalyst 11 has temporarily been poisoned with sulfur and the catalyst replacement is unnecessary.

Next, a second embodiment in which both the upstream catalyst 11 and the downstream catalyst 19 are included as degradation diagnosis objects will be described. FIGS. 9 and 10 show changes of the oxygen storage capacities of the upstream catalyst 11 and the downstream catalyst 19 between before and after the fuel is changed. In the examples shown in FIGS. 9 and 10, the change of the fuel is performed between the trip preceding the previous trip and the previous trip.

As shown in FIGS. 9 and 10, during each of the trip preceding the previous one, the previous trip, and the present trip, the oxygen storage capacities of the upstream catalyst 11 and the downstream catalyst 19 are measured. In the case of the upstream catalyst 11 shown in FIG. 9, the previous oxygen storage capacity OSC1 n-1, immediately following the change of the fuel, greatly declined from the oxygen storage capacity OSC1 n-2 preceding the previous one, similarly to the case shown in FIG. 6. However, in the case of the upstream catalyst 11, the second phenomenon, that is, the sulfur adsorption to the oxygen storage component 31, is hard to occur, and therefore substantially no further decline of the oxygen storage capacity is observed. That is, the present oxygen storage capacity OSC1 n remains substantially unchanged from the previous oxygen storage capacity OSC1 n-1.

On the other hand, in the case of the downstream catalyst 19 shown in FIG. 10, the previous oxygen storage capacity OSC2 n-1, immediately following the change of the fuel, greatly declines from the oxygen storage capacity OSC2 n-2 preceding the previous one, and farther decline is observed subsequently to the great decline. That is, present oxygen storage capacity OSC2 n further declines from the previous oxygen storage capacity OSC2 n-1. The amount of decline from the previous capacity to the present capacity is less than the amount of decline from the capacity preceding the previous one to the previous capacity, that is, there is a tendency of the oxygen storage capacity OSC2 of the downstream catalyst 19 greatly declining immediately following the change of the fuel, and subsequently continuing to gradually decline. A reason for this is that as described above, in the case where a high-sulfur fuel is fed, the downstream catalyst 19 undergoes the first phenomenon, that is, the decline of the reaction rate of the noble metal, and also the second phenomenon, that is, the sulfur adsorption to the oxygen storage component 31. Since the sulfur adsorption gradually occurs, the oxygen storage capacity also gradually declines. Considering the foregoing the oxygen storage capacity change characteristic, the catalyst degradation diagnosis is executed as follows.

Hereinafter, a catalyst degradation diagnosis process according to the second embodiment will be described with reference to FIG. 11. This process is executed by the ECU 20.

Firstly in step S301, similar to step S101, it is determined whether or not a precondition for starting the diagnosis has been satisfied. If the precondition has not been satisfied, the process ends. On the other hand, if the precondition has been satisfied, the process proceeds to step S302.

In step S302, the oxygen storage capacity OSC1 of the upstream catalyst 11 is measured. Subsequently in step S303, the oxygen storage capacity OSC2 of the downstream catalyst 19 is measured. Since the upstream catalyst 11 and the downstream catalyst 19 are diagnosis objects, an oxygen storage capacity measurement method as described above in conjunction with the second mode shown in FIG. 5 is adopted.

Subsequently in step S304, the oxygen storage capacity change amount ΔOSC1 of the upstream catalyst 11 is calculated. Subsequently in step S305, the oxygen storage capacity change amount ΔOSC2 of the downstream catalyst 19 is calculated. That is, by the method as in step S104, the oxygen storage capacity change amount of the upstream catalyst 11 ΔOSC1=OSC1 n-1−OSC1 n during the period from the previous process to the present process is calculated. Subsequently, the oxygen storage capacity change amount of the downstream catalyst 19 ΔOSC2=OSC2 n-1−OSC2 n during the period from the previous process to the present process is calculated.

After that, in step S306, the oxygen storage capacity change amount ΔOSC1 of the upstream catalyst 11 is compared with a predetermined threshold value α (see FIG. 9). If ΔOSC1>α, the process proceeds to step S307, in which the oxygen storage capacity change amount ΔOSC2 of the downstream catalyst 19 is compared with a predetermined threshold value β (see FIG. 10). If ΔOSC2>β, the process proceeds to step S308, in which it is determined that the sulfur poisoning is present regarding both the upstream catalyst 11 and the downstream catalyst 19, and simultaneously the degradation determination based on the measured oxygen storage capacity values OSC1, OSC2 of the upstream catalyst 11 and the downstream catalyst 19 is prohibited. In this manner, the sulfur poisoning of each of the upstream catalyst 11 and the downstream catalyst 19 is suitably detected, and the degradation determination regarding each catalyst based on the measured oxygen storage capacity value has been greatly lowered by the sulfur poisoning is prohibited, so that the misdiagnosis can be prevented and the diagnosis accuracy and reliability can be improved.

On the other hand, if in step S306 it is determined ΔOSC1≦α, or if in step S307 it is determined that ΔOSC2≦β, the process proceeds to step S309, in which the usual degradation determination regarding the upstream catalyst 11 and the downstream catalyst 19 is performed. That is, the measured oxygen storage capacity values OSC1, OSC2 of the upstream catalyst 11 and the downstream catalyst 19 measured in steps S102 and S103 are compared with their corresponding degradation criterion values OSC1 s, OSC2 s, respectively. If the measured value of a catalyst is greater than the degradation criterion value, it is determined that the catalyst is normal. If the measured value of a catalyst is less than or equal to the degradation criterion value, it is determined that the catalyst has degraded. For example, in the case where ΔOSC1>α regarding the upstream catalyst 11 and ΔOSC2≦β regarding the downstream catalyst 19, the usual degradation determination is performed in step S309. In this case, the possibility of melt loss is high with regard to the upstream catalyst 11, and therefore it is determined that the upstream catalyst 11 has degraded in the degradation determination of step S309. Hence, it is possible to detect a permanent degradation caused by a melt loss of one of the catalysts.

As can be understood from the foregoing description, in the diagnosis process, the presence/absence of the sulfur poisoning is detected on the basis of the measured oxygen storage capacity values obtained at two serial measurement timings. However, in another catalyst degradation diagnosis process described below, the presence/absence of sulfur poisoning is detected with regard to the two catalysts, on the basis of the measured oxygen storage capacity values obtained at three serial measurement timings.

FIGS. 12 and 13 show flowcharts of other catalyst degradation diagnosis processes according to the second embodiment. The processes are executed by the ECU 20. The process shown in FIG. 12 is executed in a cycle preceding the process shown in FIG. 13. Correspondence of the processes shown in FIGS. 12 and 13 to the examples shown in FIGS. 9 and 10 can be made as follows. For example, the process shown in FIG. 12 corresponds to a process executed during the previous trip which is shown in FIGS. 9 and 10, and the process shown in FIG. 13 corresponds to the process executed during the present trip which is shown in FIGS. 9 and 10.

Steps S401 to S407 and 9409 in the process shown in FIG. 12 are the same as steps S301 to S307 and S309 in the process shown in FIG. 11. Besides, step S408 in the process in FIG. 12 is changed in content from step S308 in the process in FIG. 11. In step S408, a sulfur poisoning detection provisional flag is set, instead of execution of the determination regarding sulfur poisoning and the prohibition of the degradation determination. That is, with regard to the upstream catalyst 11 and the downstream catalyst 19, when the oxygen storage capacity change amounts ΔOSC1, ΔOSC2 indicated in FIGS. 9 and 10 which occurred during a period from the measurement preceding the previous one to the previous measurement are greater than predetermined values α, β, respectively, the possibility of sulfur poisoning is tentatively estimated. Besides, the degradation determination is suspended, that is, is substantially prohibited.

Next, the process shown in FIG. 13 is executed. In FIG. 13, steps S501 to S503 are the same as steps S301 to S303 in the process shown in FIG. 11. After step S503, step S504 is executed, in which it is determined whether or not the sulfur poisoning detection provisional flag has been set. If the flag has not been set, the process proceeds to step S509, in which the usual degradation determination with respect to the upstream catalyst 11 and the downstream catalyst 19 is performed as in step S309.

If the flag has been set, the process proceeds to step S505, in which the oxygen storage capacity change amount ΔOSC2 of the downstream catalyst 19 from the previous process time to the present process time (the oxygen storage capacity change amount from the previous measurement to the present measurement in FIGS. 9 and 10) is calculated. Next, the process proceeds to step S506, in which the calculated oxygen storage capacity change amount ΔOSC2 is compared with a predetermined threshold value γ (see FIG. 10). If ΔOSC2≦γ, the process proceeds to step S509, in which the usual degradation determination with respect to the upstream catalyst 11 and the downstream catalyst 19 is performed.

On the other hand, if ΔOSC>γ, the process proceeds to step S507, in which it is finally determined that the upstream catalyst 11 and the downstream catalyst 19 have sulfur poisoning, and simultaneously the degradation determination with respect to the upstream catalyst 11 and the downs catalyst 19 is prohibited. Then, in step S508, the sulfur poisoning detection provisional flag is cleared. After that, the process ends.

If in the process in FIG. 12, a great decline in the oxygen storage capacity of each catalyst is detected, it is checked whether or not the oxygen storage capacity of the downstream catalyst has further declined in the process shown in FIG. 13, before the final sulfur poisoning detection is performed. Therefore, the detection accuracy can be heightened, and the diagnosis accuracy and reliability can be improved, and misdiagnosis can be more reliably prevented.

While embodiments of the invention have been described in detail above, various other embodiments of the invention can also be conceived. For example, the use and the type of the internal combustion engine are arbitrary; for example, the engine may be used for purposes other than the vehicle, and may also be of a die-injection type, or the like. The post-catalyst sensor may also be a wide-range air-fuel ratio sensor similar to the pre-catalyst sensor, and the pre-catalyst sensor used may be an O₂ sensor, similarly to the post-catalyst sensor. Sensors that detect the exhaust gas air-fuel ratio over a wide range, including a wide-range air-fuel ratio sensor, and an O₂ sensor, are referred to as “air-fuel ratio sensors”. The invention is applicable to not only three-way catalysts, but all kinds of catalysts that have oxygen storage capability.

Although FIGS. 6, 9 and 10 show examples in which the oxygen storage capacity is measured only once per one trip, this is not restrictive. The invention is also applicable to the cases where the oxygen storage capacity is measured a plurality of times per one trip, or once in a plurality of trips. Besides, the invention is applicable to the foregoing degrade diagnosis process.

While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention. 

1. A catalyst degradation diagnosis device for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst, comprising: a measurement device that measures an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and a sulfur poisoning detection device that detects sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from an oxygen storage capacity of the upstream catalyst preceding a previous oxygen storage capacity of the upstream catalyst to the previous oxygen storage capacity, an oxygen storage capacity change amount of the downstream catalyst from an oxygen storage capacity of the downstream catalyst preceding a previous oxygen storage capacity of the downstream catalyst to the previous oxygen storage capacity, and an oxygen storage capacity change amount of the downstream catalyst from the previous oxygen storage capacity to a present oxygen storage capacity of the downstream catalyst, when the oxygen storage capacity of each of the upstream catalyst and the downstream catalyst preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured by the measurement device, and the present oxygen storage capacity of the downstream catalyst has been measured by the measurement device.
 2. The catalyst degradation diagnosis device according to claim 1, further comprising: a determination device that determines the presence/absence of degradation of the upstream catalyst and the downstream catalyst based on the oxygen storage capacity measured by the measurement device; and a determination prohibition device that prohibits determination performed by the determination device when the sulfur poisoning of the upstream catalyst and the downstream catalyst is detected by the sulfur poisoning detection device.
 3. The catalyst degradation diagnosis device according to claim 2, wherein the oxygen storage capacity preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst are a value measured during a trip preceding a previous trip and a value measured during the previous trip, respectively, which are before and after refueling.
 4. The catalyst degradation diagnosis device according to claim 1, wherein the oxygen storage capacity preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst are a value measured during a trip preceding a previous trip and a value measured during the previous trip, respectively, which are before and after refueling.
 5. A catalyst degradation diagnosis device for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst, comprising: a measurement device that measures an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and a sulfur poisoning detection device that detects sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity of the upstream catalyst to a present oxygen storage capacity of the upstream catalyst, and an oxygen storage capacity change amount of the downstream catalyst from a previous oxygen storage capacity of the downstream catalyst to a present oxygen storage capacity of the downstream catalyst, when the previous oxygen storage capacity and the present oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured by the measurement device.
 6. The catalyst degradation diagnosis device according to claim 5, further comprising: a determination device that determines the presence/absence of degradation of the upstream catalyst and the downstream catalyst based on the oxygen storage capacity measured by the measurement device; and a determination prohibition device that prohibits determination performed by the determination device when the sulfur poisoning of the upstream catalyst and the downstream catalyst is detected by the sulfur poisoning detection device.
 7. A catalyst degradation diagnosis device for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst, comprising: a measurement device that measures an oxygen storage capacity of the upstream catalyst; and a sulfur poisoning detection device that detects sulfur poisoning of the upstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity to a present oxygen storage capacity, when the previous oxygen storage capacity and the present oxygen storage capacity have been measured by the measurement device.
 8. The catalyst degradation diagnosis device according to claim 7, further comprising: a determination device that determines the presence/absence of degradation of the upstream catalyst based on the oxygen storage capacity measured by the measurement device; and a determination prohibition device that prohibits determination performed by the determination device when the sulfur poisoning of the upstream catalyst is detected by the sulfur poisoning detection device.
 9. A catalyst degradation diagnosis method for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst, comprising: measuring an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and detecting sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from an oxygen storage capacity of the upstream catalyst preceding a previous oxygen storage capacity of the upstream catalyst to the previous oxygen storage capacity, an oxygen storage capacity change amount of the downstream catalyst from an oxygen storage capacity of the downstream catalyst preceding a previous oxygen storage capacity of the downstream catalyst to the previous oxygen storage capacity, and an oxygen storage capacity change amount of the downstream catalyst from the previous oxygen storage capacity to a present oxygen storage capacity of the downstream catalyst, when the oxygen storage capacity of each of the upstream catalyst and the downstream catalyst preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured, and the present oxygen storage capacity of the downstream catalyst has been measured.
 10. The catalyst degradation diagnosis method according to claim 9, further comprising: determining the presence/absence of degradation of the upstream catalyst and the downstream catalyst based on the oxygen storage capacity measured; and prohibiting determination regarding the degradation when the sulfur poisoning of the upstream catalyst and the downstream catalyst is detected.
 11. The catalyst degradation diagnosis method according to claim 10, wherein the oxygen storage capacity preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst are a value measured during a trip preceding a previous trip and a value measured during the previous trip, respectively, which are before and after refueling.
 12. The catalyst degradation diagnosis method according to claim 9, wherein the oxygen storage capacity preceding the previous oxygen storage capacity and the previous oxygen storage capacity of each of the upstream catalyst and the downstream catalyst are a value measured during a trip preceding a previous trip and a value measured during the previous trip, respectively, which are before and after refueling.
 13. A catalyst degradation diagnosis method for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst and the downstream catalyst, comprising: measuring an oxygen storage capacity of each of the upstream catalyst and the downstream catalyst; and detecting sulfur poisoning of the upstream catalyst and the downstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity of the upstream catalyst to a present oxygen storage capacity of the upstream catalyst, and an oxygen storage capacity change amount of the downstream catalyst from a previous oxygen storage capacity of the downstream catalyst to a present oxygen storage capacity of the downstream catalyst, when the previous oxygen storage capacity and the present oxygen storage capacity of each of the upstream catalyst and the downstream catalyst have been measured.
 14. The catalyst degradation diagnosis method according to claim 13, further comprising, determining the presence/absence of degradation of tile upstream catalyst and the downstream catalyst based on the oxygen storage capacity measured; and prohibiting determination regarding the degradation when the sulfur poisoning of the upstream catalyst and the downstream catalyst is detected.
 15. A catalyst degradation diagnosis method for an internal combustion engine equipped with an upstream catalyst and a downstream catalyst in an exhaust passageway of the internal combustion engine which diagnoses presence/absence of degradation of the upstream catalyst, comprising: measuring an oxygen storage capacity of the upstream catalyst; and detecting sulfur poisoning of the upstream catalyst based on an oxygen storage capacity change amount of the upstream catalyst from a previous oxygen storage capacity to a present oxygen storage capacity, when the previous oxygen storage capacity and the present oxygen storage capacity have been measured.
 16. The catalyst degradation diagnosis method according to claim 15, further comprising: determining the presence/absence of degradation of the upstream catalyst based on the oxygen storage capacity; and prohibiting determination regarding the degradation when the sulfur poisoning of the upstream catalyst is detected. 